In the initial portion of the forming section of a papermaking machine, an unsupported jet of highly aqueous stock is ejected from the head box slice onto the surface of a moving forming fabric. The unsupported jet will typically traverse a distance of from about 6 cm to about 40 cm before impinging the surface of the forming fabric at the point of impingement. The angle of impingement, α, formed between the stock jet and the plane of the forming fabric at the point of impingement has typically been from about 40° to about 10°. It is well known that improved paper formation can be obtained by minimizing both the angle α and the length of the unsupported free jet. As the angle α increases, the magnitude of the pressure exerted by the jet on the surface of the forming fabric also increases.
Impingement angles greater than about 5° have generally been found to create peak impingement pressures that may cause sheet marking, low retention of papermaking fines and fillers, and plugging of the forming fabric. Therefore, the angle α should be made as small as possible so that, ideally, the unsupported stock jet impinges on the fabric substantially tangentially.
As the unsupported length of the free stock jet increases, its outside surface begins to break up into ridges and furrows, which will eventually cause sheet basis weight variations. Further, finely dispersed fibers in the stock start to reflocculate rapidly in the unsupported jet prior to the point of impingement. Therefore, the unsupported stock jet should be made as short as possible to minimize these effects.
Due to the competing space requirements of both the head box slice lip structure and the adjacent upstream rolls such as a breast roll or forming roll, it is difficult to shorten the unsupported stock jet length without increasing the angle α. Even if the head box slice lips can be located so that the angle α is very small, and the free jet is nearly tangential to the forming fabric, air that is trapped in the small wedge shaped space between the surface of the forming fabric and the surface of the unsupported stock jet becomes entrained into the stock, forming bubbles which are detrimental to sheet formation.
Several proposals have been made to overcome these difficulties. Nelson et al, U.S. Pat. No. 3,440,136 discloses a method of avoiding air entrainment by evacuating the air from the forming zone, and flooding this area with water. However this proposal has been found to be difficult to realize in practice. Irwin et al., U.S. Pat. No. 4,734,164, disclose a forming board for a single fabric machine in which the first blade is slightly curved to permit the breast roll to be lowered slightly. The difficulty with this proposal is that air is trapped in the shallow wedge space between the jet and the forming fabric as it passes over the unvented, solid surface of the curved first blade. This air is forced into the stock as bubbles which cause formation defects. Malashenko, U.S. Pat. No. 4,802,954, discloses a lead-in blade located ahead of the curved blade element proposed by Irwin et al., which is said to reduce the amount of fluid pumped by the forming fabric into the wedge shaped space between the jet and the fabric. However, a small wedge shaped air space remains. The pressure in this space is controlled by a vacuum pump to reduce the jet disturbance. Ewald in U.S. Pat. No. 5,084,138 addresses the problem of excessive free jet length by using curved turning bars to replace large diameter breast rolls and a solid curved blade, but does not avoid air entrapment at the wedge between the jet and the fabric.
Fujiwara in U.S. Pat. No. 4,425,188 uses longitudinal spaced grooves in a twin wire forming shoe to generate cross-directional flows to realign the fibers in the incipient web so as to reduce machine direction fiber alignment. Wildfong et al., in U.S. Pat. No. 6,372,091, addresses the improved removal of water by the use of grooves in the face of a forming shoe.
However, although it is known that grooving the forming shoe, also known as an impingement shoe, may provide quality improvements in the resulting paper, it has been found that there are problems associated with the use of such grooves. Firstly, the grooves have been found to fill quickly with stock and thus substantially lose their advantageous effect, because there is no positive means to remove quickly the drained fluid from the fabric contact surface. Secondly, it has been found that the grooves may create a foiling effect between the machine side of the fabric and the groove interior, creating a level of suction sufficient to cause a deterioration in paper quality. Thirdly, it is difficult, time consuming and thus costly to create precisely dimensioned grooves in the typical ceramic surfaces of these blades, and the integrity of the ceramic can be compromised by the machining process.
The following terms have the meanings indicated below. Other terms used herein have the meanings normally associated with them in the papermaking industry.
Cross-machine direction (CD) is a direction essentially within the plane of the paper sheet and substantially perpendicular to the machine direction (MD).
Downstream is a direction in the papermaking machine essentially facing towards the reel where the finished paper product exits the machine.
Fabric support element is any stationary element in a papermaking machine over which at least one of the papermaking machine fabrics passes in sliding contact, including, but not limited to, shoes, blades, foils and agitators in the forming section, and Uhle box covers in the press section.
Laminar segments are relatively thin components, having a thickness ranging from about 1 mm (0.04 inch) or less to about 254 mm (1 inch) or more which may be assembled in a side-by-side relation in order to build up a fabric support element of a desired width. Laminar segments are typically formed of ceramic or other abrasion resistant materials.
Machine direction (MD) is a direction in the papermaking machine substantially parallel to the overall direction of travel of a paper sheet.
Machine side (MS) surface is the side of a fabric or fabric support element which is opposite to the paper side surface and is not in contact with the paper sheet.
Paper side (PS) surface is the side of a fabric or fabric support element which is either in contact with, or facing, the paper sheet being made on and conveyed through the papermaking machine.
Upstream is a direction in the papermaking machine which is essentially facing towards the headbox.
Vented describes the presence of open passageways which pass from the paper side surface through to the machine side surface of a support element, i.e. from the fabric contacting side through to the opposite side, the passageways being referred to herein as flow-through vents.
It has now been found that some of these aforementioned difficulties can be completely or substantially eliminated by the use of vents, which pass completely through the impingement shoe and thus enable the effective removal of sufficient fluid so as to prevent filling of the grooves. These flow-though vents are open from the paper side surface through to the machine side surface of the impingement shoe, and enable the removal of a greater amount of fluid, together with air which would otherwise be entrapped between the forming fabric and the stock jet, from the zone of initial impingement on the impingement shoe than has previously been possible with grooved surfaces such as defined by Wildfong et al. in U.S. Pat. No. 6,372,091.
It has further been found that these flow-through vents allow for significant variations to both the angle of impingement of the stock jet and the resultant position of the point of impingement, without any appreciable adverse effect on the resultant paper quality. This result is very surprising since, previously, variations in the angle of impingement of the stock jet by as little as one-half of one degree from an optimum value could produce significant deleterious effects on sheet quality with respect to sheet marking, retention and formation. The present invention thus allows a broader range in the angle of impingement of the stock jet than has previously been practicable.
It has also been found that these flow-through vents can be provided most advantageously by constructing the impingement shoe from a plurality of relatively thin laminar segments which are mounted together on a suitable retaining means in the papermaking machine so as to be essentially parallel to and in contact with one another and oriented at a substantially constant angle to the machine direction (MD). These laminar segments are preferably shaped from a suitable material, such as a ceramic, so as to have a machine side surface which is adapted to be mounted on a retaining means, a paper side surface which in use faces the forming fabric, an upstream surface (or leading edge) and a downstream surface (or trailing edge) and substantially parallel lateral mating surfaces facing the cross-machine direction (CD) in use. The PS surfaces of at least a first set of the segments together define a fabric contact surface of the impingement shoe which is intended to support the forming fabric. The surface may have any desired profile which would be suitable for the prevailing paper making requirements. A second set of segments, whose PS surface profile may be different from that of the first set, may be located at suitable intervals in the CD between pairs of first segments. Preferably, the second set of segments are shaped so as to form openings in the fabric contact surface of the element and enable the definition of vents which will be open from the PS surface through the MS surface. Alternatively, spacing means can be integrally constructed on one or both of the lateral CD surfaces of the first set of segments only, in which case the impingement shoe will generally be comprised of only such first segments. It has been found that the spacing means, either integrally constructed with the first set of segments, or comprising second segments, can have a suitable configuration to optimize the removal of fluid and entrapped air from the stock jet.
It has also been determined that it is possible to assemble a segmented fabric support element intended for use in any section of a papermaking machine using a plurality of at least first and second laminar segments that are shaped so as to be mounted on a suitable retaining means and located as required in any of the forming section, press or dryer sections of a papermaking machine. These first and second laminar segments are mounted on known retaining means so as to be essentially parallel to and in contact with one another and are oriented substantially in the MD or at a constant angle thereto. The construction and configuration of the segments are substantially as described above in relation to an impingement shoe, with the PS surfaces of at least the first segments together defining a fabric contact surface which is intended to support a papermaking fabric. The PS surfaces of at least the first and second segments are shaped so as to provide the PS surface of the element with any desired profile when assembled which would be-suitable for the prevailing paper making conditions. The second set of segments can be located as either spacing or profiling means at suitable intervals in the CD between selected pairs of first segments to enable the definition of openings, which may, but do not necessarily, include vents which will be open from the PS surface though the MS surface of the element. The PS surface of the second and first segments can be shaped so as to provide the assembled element with either or both a desired MD and CD profile. Third or fourth segments, or more, can also be used in a like manner in combination with the first and second segments.
The present invention seeks to provide a vented impingement shoe for use in a papermaking machine, having a plurality of flow-through vents extending from a profiled upper surface and through the lower surface of the impingement shoe.
The present invention seeks to provide a segmented fabric support element for use in the impingement zone of the forming section of a high speed paper making machine, which element is comprised of a plurality of laminar segments. The PS surfaces of the segments are shaped so as to contribute to the overall desired profile of the fabric contacting surface or PS of the element, and vents through the element, if desired, are defined by openings between selected laminar segments.
Thus, an impingement shoe constructed according to the invention performs four principal functions. Firstly, it serves to support the forming fabric as it enters the impingement zone, and secondly it serves to vent at least a substantial proportion of any air which becomes trapped in the wedge shaped space between the surface of the forming fabric and the surface of the stock jet. Thirdly, it permits drained water to be removed efficiently and speedily, and fourthly it provides a previously unattainable tolerance in the location of the point of impingement and the angle of impingement.
Further, the present invention seeks to provide a stationary fabric support element for a papermaking machine, which element is comprised of a plurality of at least first and second relatively thin laminar segments.
Further, the present invention seeks to provide segments for use in an impingement shoe or other stationary fabric support element, such that the PS surfaces of at least some of the segments contribute to a profiled fabric contact surface of the element, and vents through the element can be defined by openings between at least some of the segments. The segments are shaped so that the fabric support element can be mounted in a releasably secure manner in a suitable retaining means.
Still further, the invention seeks to provide a forming section for a papermaking machine, which forming section includes a segmented fabric support element including flow-through vents as an impingement shoe.
Still further, the invention seeks to provide a method of making such an element assembled from a plurality of segments.